In many sewer and storm water drainage systems, modular parts of the system are constructed off site, transported and installed in a prepared site. Such preparation usually consists of excavation of an area to allow a contractor to quickly install and connect the prefabricated elements of the system. Such prefabrication reduces the costs of the system in terms of the portions used and the cost of installation. However, the design of the system cannot be easily modified due to the size of the prefabricated portions and the necessity to establish safe flow conditions to prevent wastewater overflow and local flooding. One particular difficulty in municipal drainage systems is accommodating short term increased flow due to storms of infrequent but large magnitude in terms of rainfall over a short interval. These are particularly difficult to manage where solid waste material must be impeded that is often included in the flow through the system. The problem becomes critical in storm events where the waste treatment facility is unable to handle the overflow and diversion of the flow is effected to prevent backup flooding. A particular problem exists with respect to handling floating debris and waste material in such systems. Under normal flow conditions, the waste water will be fed to a treatment plant. During or after a heavy rainfall, or in winter time, a rapid melt of snow and ice, water flow will increase and, in most systems, the overflow is diverted to rivers or lakes along with the contaminating waste.